Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- And Ga-face AIGaN/GaN heterostructures

Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face Al_xGa_1-xN/GaN/Al_xGa_1-xN and N-face GaN/Al_xGa_1-xN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15<x<0.5) and thickness between 20 and 65 nm demonstrate the important role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces. Characterization of the electrical properties of nominally undoped transistor structures reveals the presence of high sheet carrier concentrations, increasing from 6×10¹² to 2×10¹³ cm^-2 in the GaN channel with increasing Al-concentration from x = 0.15 to 0.31. The observed high sheet carrier concentrations and strong confinement at specific interfaces of the N- and Ga-face pseudomorphic grown heterostructures can be explained as a consequence of interface charges induced by piezoelectric and spontaneous polarization effects.